This invention relates to an image forming apparatus such as a copying machine and a printer and, more particularly to an image forming apparatus capable of detecting an imaging performance of a photosensitive member by the use of a sensor arranged in the vicinity of and opposed to a rotating surface of the photosensitive member.
In a copying machine, printer, or like image forming apparatus, it is extremely important in setting imaging conditions including a charged amount and an exposure amount to detect and grasp an imaging performance of the photosensitive member. Generally, data indicative of the imaging performance of the photosensitive member include the density of a toner image formed on the photosensitive member (toner density), a level of potential on the surface of the photosensitive member charged by a charger, etc.
In conventional image forming apparatuses, the toner density is detected by sensor arranged in a position in the vicinity of and opposed to the circumferential surface of the photosensitive member rotating at a constant speed. This sensor detects the toner density of the toner image in a specified region after the toner image is formed on the photosensitive member.
Here will be described the detection of the toner density of the toner image formed on the photosensitive member in the conventional image forming apparatus with reference to FIGS. 17A to 17C. FIG. 17A is a graph showing a sensor output over time when the toner density is detected while the photosensitive member is stationary. FIG. 17B is a graph showing a sensor output over time when the toner density is detected while the photosensitive member is rotating. FIG. 17C is a graph showing a sensor output when a sampling is made continuously so as to measure the toner density while the photosensitive member is rotating.
When the photosensitive member is stationary, the toner density is detected at an arbitrary point on the surface of the photosensitive member. Accordingly, the sensor output varies only due to variations in detection performance and output performance of the sensor itself. Thus, the sensor output does not vary greatly as seen from FIG. 17A. Contrary to this, when the photosensitive member is rotating, other detecting conditions change in addition to the variations of the sensor itself. These conditions include a change in a distance between the photosensitive member and the sensor due to the nonuniformity of the surface of the photosensitive member and according to the rotation of the photosensitive member, etc. The changes in these conditions cause the change in the sensor output over time as shown in FIG. 17B. It will be appreciated that the photosensitive member makes one turn during a period defined by time t12 and time t13 in FIG. 17B. Accordingly, if the toner density is measured a plurality of times (e.g. 100 times) in the above conventional image forming apparatuses, the sensor output varies greatly as shown in FIG. 17C.
In the conventional image forming apparatuses, when an optical system is caused to scan the same document a plurality of times, the optical system is controllably driven lest it should start an exposing operation from the same position with respect to rotating direction of the circumferential surface of the photosensitive member so as to prevent the deterioration of the surface of the photosensitive member, or the like. In other words, even when the toner density is measured a plurality of times using the same document, the exposure start position on the photosensitive member is not fixed. Thus, in the conventional image forming apparatuses in which the toner density is measured in the specified region on the photosensitive member, the toner density may not be necessarily measured at the same position on the photosensitive member. Thus, the measurement values may vary due to the fact that the measurement is made at different positions.
In view of the foregoing, when the toner density is measured while the photosensitive member is rotating, it is necessary to handle as errors the variations in the measurement values due to the different measurement positions in addition to the variations of the sensor itself. Accordingly, it is extremely difficult to set the respective imaging conditions so as to provide stable images in accordance with the toner density detected in the conventional image forming apparatuses.
Likewise, even when the potential is measured so as to use the potential on the surface of the photosensitive member charged by the charger as a data indicative of the imaging performance of the photosensitive member, the measurement values varies due to the different measurement positions.